The present disclosure relates to a simply fabricable small zeroth-order resonant antenna with extended bandwidth and high efficiency, and more particularly to, a zeroth-order resonant antenna applicable to wireless communication devices because its resonant frequency is determined regardless of the size of an antenna and the resonant antenna has extended bandwidth in spite of its small size.
Metamaterials that have been extensively studied in regard to microwave circuits and antennas are artificially synthesized to show special electromagnetic characteristics that are rarely observed in nature. Compared to existing natural materials, the metamaterials have special characteristics such as anti-parallel phase, group velocities, and zero propagation constant, and may be implemented by Split-ring Resonator (SSR) or Composite Right/Left Handed Transmission Line (CRLH TL).
CRLH TL may be applied to a dominant mode leaky-wave antenna that radiates in forward and backward directions by using the characteristics of anti-parallel phase and group velocities. Also, in regard to left-handed material characteristics, a resonator has an infinite wavelength by a zero propagation constant, and the resonant frequency is independent of the size of the resonator. Accordingly, the zero propagation constant characteristics of the resonant antenna enables further miniaturization of the resonant antenna compared to a related-art half-wavelength antenna.
FIG. 1 is a diagram illustrating a related-art zeroth-order resonant (ZOR) antenna. Referring to FIG. 1, the related-art ZOR antenna (hereinafter, referred to as ‘prior art 1’) may include a plurality of unit cells, each of which has a size of 7.3×15 mm2. Also, w1 may equal 15.0 mm, and w2 may equal 0.2 mm in FIG. 1. When the antenna of FIG. 1 is configured with two unit cells, the resonant frequency is 3.38 GHz, and the electrical size of the antenna becomes λ0/6×λ0/6×λ0/57 with respect to the resonant frequency f0. Accordingly, as the ZOR antenna uses a zero propagation constant, the ZOR antenna has an effect of size reduction compared to a related-art antenna. However, since the ZOR antenna according to the prior art 1 shows a bandwidth of 0.1% or less, there is a limit to its application to wireless communication apparatuses.
In recent years, studies have been conducted to solve a bandwidth limitation of ZOR antennas. FIG. 2 is a diagram illustrating a configuration of a metamaterial ring antenna proposed for bandwidth improvement. The ring antenna (hereinafter, referred to as ‘prior art 2’) of FIG. 2 may be implemented over a multi-layer structure that includes a thick substrate having a low permittivity. The substrate is supported by a support bracket, and the bandwidth increases to 6.8% by a sleave balun. However, there is a limitation in that it is not easy to manufacture.
As an alternative, the bandwidth of the ZOR antenna increases by strip matching ground. In this case, the fractional bandwidth of the antenna is improved by 8%. The ZOR antenna may also be manufactured in a multi-layer substrate including thin substrates of high permittivity that are stacked on a thick substrate of low permittivity. Another method for solving the bandwidth limitation is to have two resonant frequencies adjacent to each other. Such an antenna includes two resonators having minutely different resonant frequencies. In this case, the bandwidth increases by 3.1%.
FIG. 3 is a diagram illustrating a structure of another small-size antenna with extended bandwidth. Each of unit cells constituting the antenna (hereinafter, referred to as ‘prior art 3’) shown in FIG. 3 includes an upper patch, a vertical via connecting the upper patch to a ground, and four metal-insulator-metal (MIM) capacitors overlapping adjacent unit cell. Also, a ground plane is divided into an antenna ground, a microstrip feeder ground, and a strip matching ground. Detailed description of dimension of each part will be omitted in FIG. 3. Through the above configuration, the antenna of the prior art 3 may achieve broader impedance matching and smaller size compared to a related-art antenna. However, there is a limitation in that the antenna of the prior art 3 also requires a multi-layer structure in which thin substrates of high permittivity are stacked on a thick substrate of low permittivity, similarly to the prior art 2.
Necessity of development of a small-size antenna having a simple structure and showing extended bandwidth and high efficiency compared to the above-described prior arts is being proposed together with development of portable devices that are gradually miniaturized.